1. Field of the Invention
The present invention relates generally to the field of optical imaging and more particularly to catadioptric optical systems used for microscopic imaging, inspection, and lithography applications.
2. Description of the Related Art
Many optical systems have the ability to inspect or image features on the surface of a specimen, such as inspecting defects on a semiconductor wafer or photomask, or alternately examining a biological specimen on a slide. Microscopes have been used in various imaging situations, including biology, metrology, semiconductor inspection, and other complex inspection applications where high resolution images of small areas and/or features are desired.
Typically available imaging systems include microscopes, which use offer inspection using dry imaging, or imaging in a gaseous medium such as air. Many newer applications either require or may benefit from immersion imaging. Unlike dry imaging, immersion imaging immerses the sample in water or other liquid and images or inspects the image within the liquid. Immersion imaging can, in certain circumstances, provide increased optical properties, including but not limited to enhanced resolution, over dry imaging. Furthermore, due to the nature and fragile properties associated with certain biological specimens, biological imaging systems frequently cannot use dry imaging whatsoever. In this situation, a biological imaging system can only image a sample while the sample is immersed in liquid, necessitating the use of a system able to perform immersion imaging.
Problems with immersion imaging and objectives employed within immersion imaging systems include the ability to resolve the image using immersion imaging operation in the presence of either low wavelength or broad wavelength range light energy, successfully employing different types of illumination and imaging modes, and effective objective usage in widely available standard equipment, such as microscopes.
Microscopes designed to support immersion imaging can be difficult to design for high resolution specimen imaging. To improve the quality of the image received, such a system may use various imaging modes to enhance the appearance of desired features on the specimen. Imaging modes used in an immersion imaging system may include bright field, dark field, differential interference contrast, confocal, and other imaging modes offering different benefits depending on the type of specimen, the features on the specimen being observed, the imaging environment, and other related considerations.
Certain imaging modes may employ light energy of varying wavelengths, and thus the ability to effectively resolve images and operate in the presence of a wide variety of wavelengths and over various wavelength ranges may be particularly beneficial. For immersion imaging, the system objective is one of the most critical components of the design and may use light having a broad range of wavelengths, including wavelengths below 400 nm. Some available UV objectives can transmit light at wavelengths down to a wavelength of 340 nm, but these objectives do not provide accurate imaging performance for light wavelengths below the range of approximately 400 nm. These types of objectives are mainly used for fluorescence, where wavelengths from 340 nm through the visible light spectrum excite fluorescence components in marker dyes. The fluorescent emission for these objectives is typically in the visible light spectrum, so imaging performance in the visible light spectrum is the specific type of performance required. Such fluorescence excitation does not perform an inspection of the specimen and thus such an objective provides limited, if any, inspection functionality.
The ability for an objective to operate within a standard microscope is both desirable and difficult to achieve for the detailed inspection performance required for semiconductor and biological inspections discussed above. No immersion objectives are currently known that can support broad band inspection performance at light wavelengths below 400 nm. Some dry objective designs may be highly corrected for broad band imaging at wavelengths below 400 nm, but none of these dry objective designs can be used in a standard microscope system. The dry objective designs are typically too large, have insufficient numerical aperture (NA), or have an insufficient field size.
It would therefore be beneficial to provide a system and objective for use in conjunction with standard microscopes and microscopy applications that overcome the foregoing drawbacks present in previously known dry imaging/immersion imaging systems. Further, it would be beneficial to provide an optical inspection system design having improved functionality over devices exhibiting the negative aspects described herein.